ABSTRACT Support is requested for a Keystone Symposia meeting entitled Cell Plasticity within the Tumor Microenvironment, organized by Drs. Sergei Grivennikov, Florian R. Greten and Mikala Egeblad. The meeting will be held in Big Sky, Montana from January 8-12, 2017. During the last decade, it has become unequivocally evident that tumor development is not a cell-autonomous process but rather depends on the intricate reciprocal interplay of mutant tumor cells with their local and distant environments. Composition and polarization of cells in the tumor microenvironment depends on genetic as well as environmental factors and is directly influenced by tumor therapy. Indeed, therapies that aim to shape the local immune milieu and consecutive signaling pathways in both stromal and tumor/stem cells address the complex pathophysiology of tumors more adequately and may therefore add substantial benefits for therapy. An absolute pre-requisite for such an endeavor is a comprehensive understanding of the exact molecular basis of the complex signaling networks in the tumor microenvironment that control the plasticity of both stromal and tumor cells, thereby shaping the complex cellular contexture, which ultimately forms a pro- or anti-tumorigenic milieu. Thus, this symposium aims to gather a comprehensive functional understanding of mediator-dependent cellular and molecular events that are responsible for the plasticity of both stromal and tumor/stem cells. It will bring together experts in cancer, stromal and immune cells to synthesize scientific knowledge about the phenomenon of cell plasticity within the tumor microenvironment, to define molecular and cellular pathways mediating plasticity and to propose approaches to interfere with cell plasticity for a new generation of effective therapeutic approaches in cancer and chronic injury. Importantly, the meeting will bring together interdisciplinary groups of scientists or investigators who normally would not have an opportunity to meet (classical cancer biologists, tumor immunobiologists, stem cell experts and scientists working on stromal cells).